Water absorbing resins have found wide use in sanitary goods, hygenic goods, water retaining agents, dehydrating agents, sludge coagulants, thickening agents, condensation preventing agents and release control agents for various chemicals. Water absorbing resins heretofore known include hydrolysis products of starch-acrylonitrile graft polymers, carboxymethylcellulose, cross-linked polyacrylate products and other resins such as polyvinyl alcohol, polyethylene oxide and polyacrylonitrile resins. Of these water absorbing resins, the hydrolysis products of starch and acrylonitrile graft polymers have comparatively high ability to absorb water but require a cumbersome process for production and have the drawbacks of low heat resistance and decaying or decomposing easily due to the presence of starch.
One of the processes for polymerizing acrylic acid and acrylates is aqueous solution polymerization. The polymer obtained by this process is soluble in water and, therefore, is cross-linked to modify the polymer into a useful water absorbing resin. However, even if the modification is effected by reacting a cross-linking agent concurrently with or after aqueous solution polymerization, the resulting reaction product is in the form of a highly viscous aqueous solution of a gel containing absorbed water which is difficult to handle. Thus, the aqueous solution or gel must be dehydrated (dried) to obtain a water absorbing resin in the desired solid or powder form. It is nevertheless difficult to dry the reaction product efficiently by the usual rotary drum roller method or spray drying method because care must be taken to avoid excessive cross-linking which results from overheating during drying and insufficient drying results in reduced cross-linking density. Extreme difficulties are therefore encountered in preparing a product of a desired low water content and good water absorbing ability.